Question

Find the reference number of 11pi/9

Answer 1

Unsure of what you mean by "reference number."  

I do recognize that 11pi/9 is in quadrant III.  I suspect that the so-called "reference number" is the acute angle between the end ray of 11pi/9 and pi, or, in more easily comparable numbers, between 9pi/9 and 11pi/9.

That difference is 2pi/9.